The present invention relates to a fluid power transmission system such as a fluid coupling or torque converter for transmitting a torque through a fluid such as oil and, more particularly, to a fluid power transmission system provided with a lock-up clutch for connecting a drive side member and a driven side member directly.
A lock-up clutch to be mounted in a fluid power transmission system such as a torque converter is used to eliminate the power loss in case of transmitting the torque through a fluid. For example, a torque converter for a vehicle is arranged between the inner surface of a front cover and a turbine runner so that the torque may be directly transmitted from the front cover to an output shaft by pushing the lock-up clutch into engagement with the inner surface of the front cover. If this lock-up clutch is engaged, the torque is transmitted not through the fluid so that the torque transmission efficiency is substantially 100% without any power loss. However, the fluctuations of the engine torque are transmitted as they are to the output shaft. In case the fluctuations of the engine torque are high, the resultant vibrations are transmitted through the output shaft to a drive mechanism such as an automatic transmission to establish torsional resonations. As a result, the vibrations are transmitted through the mount to the body to vibrate the body panel and the floor panel. Then, there arises a defect that the so-called "booming noise" is generated in the car compartment.
In order to eliminate this defect which is caused by engaging such lock-up clutch, there has been proposed in the prior art a torque converter provided with the following means.
FIG. 45 shows a torque converter 1 which is provided with a lock-up clutch, as disclosed in Japanese Utility Model Laid-Open 157746/1986. A housing 2 connected to the (not shown) output shaft of the engine is constructed of a front cover 2a and the casing of a pump impeller 3. In the housing 2, there are disposed a stator 4 and a turbine runner 5. The stator 4 is held through a one-way clutch 4a, and the turbine runner 5 is connected to the output shaft 6 through a turbine hub 6a. Between the front cover 2a and the turbine runner 5, on the other hand, there is arranged a lock-up clutch 7. This lock-up clutch 7 is composed of: a disc-shaped driven member 8 which is fixed on the turbine hub 6a by means of rivets 13 so that it may rotate integrally with the turbine runner 5; and a disc-shaped drive member 10 which is arranged between the inner surface of the front cover 2a and the driven member 8 and which is interleaved relatively rotatably and axially movably on the outer circumference of the turbine hub 6a. Moreover, the driven member 8 is provided at its outer circumference with damper springs 9 for damping the driven member 8 and the drive member 10. The opposed surfaces of the driven member 8 and the drive member 10 are formed at their portions close to the center with annular projections 8a and 10a which are made so concentric that they can be interleaved on each other at a predetermined clearance. These projections 8 a and 10a shear the AT oil, which is confined in between, when the driven member 8 and the drive member 10 rotate relative to each other, so that they establish a resistance acting as a force for attenuating the vibrations in the torsional direction. Here is provided a viscous attenuation mechanism 11.
When the running speed increases so that the vehicle state reaches the lock-up range, the oil pressure between the front cover 2a and the drive member 10 is set at a lower level than that of the oil pressure at the side of the turbine runner 5. Then, the drive member 10 is pushed by the pressure difference toward the front cover 2a and forced to contact with a friction member 12 which is fixed on the inner surface of the front cover 2a. Then, the lock-up clutch 7 is engaged so that the power is transmitted from the housing 2 through the drive member 10 and the damper springs 9 and further from the driven member 8 to the output shaft 6. If the lock-up clutch 7 is thus engaged, a part or most of the rotational torque of the engine is mechanically transmitted to the output shaft 6 not through the fluid. If the engine torque fluctuates in this state, the damper springs 9 connecting the drive member 10 and the driven member 8 are tensed or compressed in accordance with the input torque. As a result, the vibrations caused by the torque fluctuations are reduced by the damper springs 9, and the booming noise can be prevented by reducing the spring constants of the damper springs 9.
In the torque converter 1 thus far described, on the other hand, the damper springs 9 usually acting as vibration absorbing elements increase the vibrations under a special situation having a stepwise change in the input torque and may cause the so-called "surging phenomena". However, this surging phenomena can be prevented to provide an excellent driving feel at a lock-up time, because the viscous attention mechanism 11 arranged in parallel with the damper springs 9 acts to suppress the relative rotations of the drive member 10 and the driven member 8.
In the torque converter 1 of the prior art thus far described, however, the driven member 8 is provided with the damper springs 9 at its portion closer to the outer circumference and with the viscous attenuation mechanism 11 at its portion closer to the center. Thus, the torque converter 1 has the following problems.
The damper springs 9 are required to absorb the vibrations caused by the high fluctuations of the input torque and is spatially restricted by their aforementioned arrangement. In the prior art, therefore, the number of damper springs had to be decreased, and their respective spring constants also had to be increased. As a result, the vibrations having a relative frequency cannot be absorbed so that they are transmitted to the power transmission mechanism such as the automatic transmission to cause the booming noise.
One of the causes for generating the booming noise is that the spring constants of the damper springs 9 have large spring constants. In order to reduce the booming noise, it is conceivable to reduce the spring constants of the damper springs 9. In case, however, the spring constants of the damper springs 9 are reduced, the relative rotational angle of the drive member 10 and the driven member 8, i.e., the torsional angle is increased to enlarge the overall compression of the damper springs 9. This makes it necessary to increase the number of damper springs 9 or to elongate the individual damper springs 9 and accordingly to retain the space therefor. In case, on the other hand, the spring constants are decreased, the drive member 10 and the driven member 8 are highly twisted relative to each other even for a small change in the input torque. After this, the damper springs 9 release the energy so that the drive member 10 and the driven member 8 are highly twisted relative to each other in the opposite directions. As a result, the surging phenomena are liable to occur. Thus, it is necessary to enhance the attenuation characteristics of the aforementioned viscous attenuation mechanism.
If the spring constants of the damper springs 9 are thus reduced, the structures of the damper mechanism and the viscous attenuation mechanism 11 have to be accordingly changed from those of the prior art shown in FIG. 45. In the existing structure, in which the damper springs 9 are circumferentially arranged in one row on the circumference of the driven member 8 and which the viscous attenuation mechanism 11 is arranged circumferentially inside of the damper springs 9, there are independently necessary the space for fitting the numerous damper springs 9, the space for the rivets to hold the damper springs 9, and the space for mounting the viscous attenuation mechanism 11. It is, however, practically difficult to retain such wide spaces in the restricted inside space. After all, the restriction on the space makes it impossible to reduce the spring constants of the damper springs 9 sufficiently.
In the aforementioned torque converter 1 of the prior art, the drive member 10 is so fitted on the outer circumference of the turbine hub 6a as to move in the axial direction so that it may engage or release the lock-up clutch 7. On the contrary, the driven member 8 for forming the viscous attenuation mechanism 11 together with the drive member 10 is fixed integrally with the turbine runner 5 on the outer circumference of the turbine hub 6a by means of the rivets 13.
When the lock-up clutch 7 is engaged, the drive member 10 moves toward the front cover 2a while leaving the driven member 8 immovable. As a result, these two members 10 and 8 are apart from each other to reduce the axial overlapped length of the projections 10a and 8a of the viscous attenuation mechanism 11. Thus,there arises a problem that the viscous torque to be generated drops.
In the torque converter 1 of the prior art thus far described, moreover, the driven member 8 is at its portion closer to the outer circumference provided with the damper springs 9 and its portion closer to the center with the projections 8a and 10a of the viscous attenuation mechanism 11. Since these projections 8a and 10a are formed concentric and annular, their relative rotations never fail to establish the viscous torque. Even if the springs constants of the damper springs 9 are reduced, for example, the vibration attenuation can always be established to prevent the surging phenomena. In the aforementioned structure, however, the individual projections 8a and 10a are always positioned close to and facing each other through the oil. As a result, the fine vibrations having a relatively high frequency are transmitted from the drive member 10 to the driven member 8 through the individual projections 8a and 10a and the oil confined inbetween, thus causing a problem that the booming noise is serious.
On the other hand, the vibration attenuation by the viscous attenuation mechanism 11 is established as a result that the viscous fluid or oil is caused to absorb the kinetic energy in terms of the thermal energy by sharing the oil. In the structure of the prior art, therefore, the energy for rotating the drive member 10 and driven member 8 relative to each other is partially absorbed at all times. Thus, there arises a disadvantage that the energy is unnecessarily consumed to deteriorate the fuel economy of the vehicle.
On the other hand, FIG. 46 shows a torque converter which is provided with a lock-up clutch of the prior art, as disclosed in Japanese Utility Model Laid-Open No. 28944/1982. This torque converter 14 is constructed to include: a drive plate 15 connected to the output shaft of an engine; a housing 16 connected to the drive plate 15 by means of bolts; a pump impeller 17 integrally formed inside of the housing 16; a stator 19 fixed to a stationary shaft through a oneway clutch 18a; a turbine runner 20 arranged to face the pump impeller 17 across the stator 19; a disc-shaped driven member 22 fixed on a hub 21a which is so splined to an output shaft 21 as to move in the axial direction; and a disc-shaped drive member 24 so connected to the driven member 22 as to rotate integrally with the damper springs 23 and having its central side so attached to the hub 21a as to move in the axial direction. These driven member 22 and drive member 24 constitute a lock-up clutch 25 for transmitting the torque by engaging the circumferential edge of the drive member 24 with the inner surface of the front cover 16a of the housing 16.
In the torque converter 14 thus far described, too, the output torque of the engine is transmitted through the drive plate 15 to the housing 16 of the torque converter 14 so that the pump impeller 17 integrated with the housing 16 is rotated. With the lock-up clutch 25 being released, the torque is transmitted from the pump impeller 17 through the AT oil to the turbine runner 20 so that the output shaft 21, on which the turbine runner 20 is fixed through the hub 21a, is rotationally driven. Thus, the torque is transmitted through the AT oil so that the vibrations due to the fluctuations of the engine torque can be absorbed to provide a satisfactory driving feel.
If, on the other hand, the AT oil in the housing 16 has its pressure controlled to bring the circumferential edge of the drive member 24 into contact with the inner surface of the front cover 16a so that the lock-up clutch 25 is engaged, then the output torque of the engine is transmitted from the housing 16 through the drive member 22 and the damper springs 23 to the drive member 24 and further through the hub 21a to the output shaft 21. In this case, the torque is transmitted mechanically not through the liquid to the output shaft 21. Despite of this direct transmission, the vibrations to be caused by the fluctuations of the engine torque or the like are absorbed through the tensions or compressions of the damper springs 23 so that a satisfactory driving feel can be attained.
In the torque converter 14 of the prior art shown in FIG. 46, torque to be applied to the lock-up clutch 25 is high in case the torque inputted from wheels is increased, when the lock-up clutch 25 is engaged, by the undulations of the road surface. As a result, the extent of deformation of the damper springs 23 may exceed the allowable range. If this deformation extent of the damper springs 23 exceeds the allowable range, there arises a problem that the torque abruptly rises to cause the shocks.
If an excessive load is exerted upon the output shaft side (or the wheel side) while the lock-up clutch 25 being engaged so that the deformation extent frequently exceeds the allowable range, as has been described hereinbefore, the overload causes a problem that the lifetimes of the springs are shortened.
Incidentally, the damper mechanism in the torque converter must have a capacity for absorbing the maximum input torque anticipated. If the damper springs having large spring constants are used to stand the high torque, there arise defects that the vibrations cannot be sufficiently reduced and that the booming noise is intensified. Thus, the damper mechanism is required to increase the spring constants to some extent for its strength and to decrease the spring constants for reducing the vibrations. In order to satisfy these contradictory requirements, the damper mechanism is constructed in the prior art by using several kinds of damper springs having different spring constants.
This structure is exemplified in Japanese Patent Laid-Open No. 252964/1986, as will be briefly described in the following.
As shown in FIGS. 47 and 48, there is mounted in a housing 27 of a torque converter 26: a pump impeller 28; a turbine runner 29 arranged to face the pump impeller 28; a stator 30 arranged between the pump impeller 28 and the turbine runner 29; and a lock-up clutch 31 arranged between a front cover 27a and the turbine runner 29.
This lock-up clutch 31 is composed of a disc-shaped drive member 32 to be engaged with and released from the inner surface of the front cover 27a, and a disc-shaped driven member 34 arranged to face the drive member 32.
On the other hand, the driven member 34 is composed of: a first plate 34a connected to the outer circumference of the drive member 32; a second plate 34b connected to the first plate 34a through first damper springs 35; and a third plate 34c connected to the second plate 34b through second damper springs 36. The third plate 34c is fixed together with the turbine runner 29 on a hub 29a which is splined to the output shaft 33 of the torque converter 26. Moreover, the aforementioned first damper springs 35 and second damper springs 36 are given different spring constants such that the spring constants of the first damper springs 35 are set at smaller values than those of the second damper springs 36.
In the aforementioned torque converter 26 of the prior art, therefore, the first damper springs 35 are tensed or compressed to absorb the torque fluctuations, if an input torque in the lock-up state is low, and the second damper springs 36 having the larger spring constants are compressed to absorb the torque if a high torque is inputted. At a low torque time, the angle of torsion of the lock-up clutch 31 for a predetermined torque is enlarged. At a high torque time, the angle of torsion of the lock-up clutch 31 for the predetermined torque is reduced. Thus, the spring characteristics change in the two steps. As a result, the booming noise, which is caused by the fluctuations of the engine torque in the ordinary running state, can be prevented by the actions of the first damper springs 35 having the small spring constants. For a high torque inputted temporarily, on the other hand, the second damper springs 36 having the large spring constants act to prevent the damage.
The damper mechanism using the damper springs having different spring constants is advantageous in that they can stand a high socking torque, and is especially effective for a vehicle such as an off-road car, in which a high torque is relatively frequently inputted from wheels. Despite of these advantages, however, the following disadvantages are invited because the spring constants highly change across a predetermined angle of torsion.
In case impact torque is inputted, the damper springs having the smaller spring constants are at first compressed abruptly and highly. When the angle of torsion then reaches the value for changing the spring constant, then the abrupt and high torque is transmitted through the damper mechanism. As a result, the torque of the output shaft of the vehicle abruptly changes. This change is felt by the rider as such shocks as will be caused in case the drive mechanism chatters, and may deteriorate the riding comfort and the stability of the vehicle.
In the aforementioned structure of the prior art, moreover, the spring constants are changed abruptly and highly at a predetermined angle of torsion. In case high impact torque is inputted from the wheels while the vehicle is running on a rough road to compress the damper springs having the larger spring constants and to eliminate the input of the torque from the wheels, the energy stored in the damper springs having the larger spring constants is abruptly released until the changing point of the spring constants, i.e., until the angle of torsion comes to the angle at which the spring constants change. This is the situation similar to the case, in which the input torque is highly changed. These torque fluctuations are reduced by the damper springs having the smaller spring constants. As a result, the so-called "surging phenomena", in which the angle of torsion is repeatedly changed high and slowly, may be caused.
In the torque converter thus far described, the spring characteristics of the damper mechanism of the lock-up clutch is abruptly changed at the predetermined angle of torsion, to cause a disadvantage that the shocks or the surging phenomena may occur.